An avalanche photodiode (APD) is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APD's can be thought of as photo-detectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint, APD's can be regarded as the semiconductor analog to photomultipliers.
Upon application of a reverse bias “operating voltage” (Vop), APD's show an internal current gain effect due to impact ionization (avalanche effect). In general, the higher the reverse voltage, the higher the gain. A “trans-impedance amplifier” (TIA) can then be used to convert this current effect to a voltage effect.
APD's are frequently used in LASER range finders, which transmit pulses of LASER light and then detect the light echo which is reflected from a remote object. By measuring the time delay between pulse transmission and echo detection, the distance to the reflecting object can be calculated. APD's are also used in missile guidance systems, and in many other applications where the performance of the system depends strongly on the signal-to-noise performance (S/N) and/or the Responsivity-to-Noise ratio (R/N) of the APD's.
The performance of an APD-based system typically depends on the performance of the APD itself. In the absence of ambient light “noise” and impinging sunlight, optimal performance will be achieved when the Vop is adjusted such that the total APD noise is equal to the amplifier noise. In the absence of ambient light, the optimal Vop will be close to the maximum, or “breakdown” voltage (Vb) of the APD. In the presence of background light noise and/or sunlight, optimal performance will result when the Vop is set to a lower value. There is a lower voltage limit VopBW at which the APD might not meet the timing (bandwidth) required to evaluate the laser pulses.
APD's experience significant performance changes as a function of temperature, including changes to Vb and VopBW. In addition, the system level manufacturing process typically adds additional variation from one part to the next. These variations in APD characteristics can result in extreme performance variations unless compensating adjustments are made to the operating voltage of the APD (Vop). Therefore, optimal performance can only be achieved if the electronic performance of the APD is characterized and the Vop is adjusted accordingly. Depending on the operating conditions, it may be necessary to repeat this Vop adjustment as the temperature changes, and/or as background light “noise” and impinging sun conditions change. If the Vop of a plurality of APD's is provided by a common power supply, it is necessary to characterize each of the APD's and set the Vop to a value which will be optimal for the entire group.
APD's are sometimes used in missile guidance systems, such as the guidance system of the APKWS missile. In such cases, it can be necessary to periodically characterize and adjust the Vop for all onboard APD's during the flight of the missile, as light and temperature conditions change. In addition, it can be necessary to perform the characterizations and adjustments quickly, since frequent measurements are required and the missile travels very rapidly. In particular, the APKWS includes seven APD's, each of which must/be characterized before the Vop can be adjusted. A typical requirement is that an entire set of APD's must be characterized and the Vop adjusted in less than 300 ms.
Vop characterization methods typically require multiple sequential background-noise measurements so as to determine an acceptable Vop voltage. These algorithms have typically been loosely tied to the APD manufacturer's specifications, and have not always met the specified bandwidth requirement. For systems that include a plurality of APD's sharing a common Vop, these algorithms have typically required a detailed systems analysis to evaluate how the Vop should be set. In particular, these methods typically require that the breakdown voltages of each of the APD's be determined so as to find the optimal Vop solution. As a result, such characterization and adjustment methods can be time consuming and are not always optimal.
What is needed, therefore, is a method for rapidly and accurately determining the performance characteristics of one or more avalanche photo diodes (APD's), and selecting an optimal operating voltage (Vop) for the APD's.